Deletion of a portion of the long arm of chromosome 18 (18q- syndrome) is one of the most common aneusomies to affect humans. Although the size of the deletion is variable, affected children share common clinical features such as short stature and mental retardation. The hypothesis on which this study is based is that there are alterations in the timing of myelination of central and peripheral nervous systems secondary to having only a single copy of the gene for myelin basic protein; this leads to disregulation of the hypothalamic pituitary axis and underproduction of GH. The abnormal hormonal milieu further retards myelination which exacerbates neurodevelopmental abnormalities and contributes to the severity of the mental retardation. The purpose of this study is to examine whether GH treatment has beneficial effects in such areas as the rate of linear growth, hypothalamic/pituitary function, the degree of CNS myelination, hearing, and neurodevelopmental maturation.